Acceleration of the Bootstrapping in TFHE by FPGA

Privacy-preserving computing is playing an ever-increasingly important role in various fields. A leading example of privacy-preserving computing is Fully Homomorphic Encryption (FHE). FHE enables arbitrary computations directly on the ciphertext. This guarantees that the original data will not be disclosed while processing the data. However, FHE brings in the high computation cost which, in turn, limits the application of FHE. Among all steps of FHE, bootstrapping is a critical operation yet a bottleneck for the FHE efficiency. Torus FHE (TFHE) was presented as a method which can compute arbitrary Boolean functions on ciphertext with fast gate bootstrapping. In this paper, we show an implementation of TFHE gate bootstrapping on ZYNQ ZCU102 FPGA board. The memory operation is specially organized to facilitate the implementation of the adopted Number Theoretic Transform (NTT) of external product. Each function involved in the TFHE gate bootstrapping is implemented at the register-transfer level (RTL), and each operation is carefully scheduled to maximize the parallelism. Experimental results show that with ZCU102 working at the frequency of 300MHz, the proposed scheme can bootstrap one bit within 1.9ms on average. Compared with the accelerated TFHE using the mainstream CPU, the proposed scheme shows a 5.0X speedup. If under the similar clock frequency, it presents 1.23X faster than cuFHE which is accelerated by GPU. The proposed scheme also shows other advantages such as high efficiency and better tradeoff than existing FPGA-based acceleration schemes.